Energy barrier to TTC–TTT isomerisation for the merocyanine of a photochromic spiropyran

The 1H NMR signal of a proton of a spiropyran photomerocyanine was resolved at low temperature into two signals attributed to the two isomers, TTT and TTC. The equilibrium distribution and free energy difference between the TTT and TTC isomers were calculated to be 0.15 and 4.6±0.7 kJ mol−1, respectively. By 1H NMR lineshape analysis at various temperatures, an activation barrier for the TTC→TTT isomerisation of 43.6±3 kJ mol−1 was calculated.